This invention relates generally to optical fiber interferometers and more particularly to an optical fiber interferometer in the Fabry-Perot configuration operating at its maximum sensitivity.
Optical fiber interferometers developed in recent years are generally either amplitude sensing or phase sensing. Because of their greater sensitivity, however, phase sensors, especially those which employ a Mach-Zehnder arrangement, have typically been preferred. In such an arrangement, shown for example in U.S. Pat. No. 4,524,322 to Lloyd C. Bobb, a laser beam is split, with one part of the beam being transmitted by a reference fiber and the other by a sensing fiber which is exposed to the environment or field of interest. The two beams are subsequently recombined and interfere on the surface of a photodetector. Suitable means is provided on the reference fiber for either shifting the optical frequency or modulating the phase in order to detect the original phase modulated signal. While such Mach-Zehnder type optical fiber interferometers provide greatly enhanced sensitivities, their relative complexity of design (i.e. the requirement for a reference fiber, beam splitters and combiners, etc.) results in a more costly, difficult-to-fabricate interferometer having an increased number of sources of noise due to the additional components required.
Single fiber interferometers have also been developed. Such an interferometer in the Fabry-Perot configuration is discussed in U.S. Pat. No. 4,536,088 to Rashleigh et al, and described in an article by S. J. Petuchowski et al (IEEE Journal of Quantum Electronics, Vol. QE-17, No. 11, November 1981, p. 2168) and in another article by Yoshino et al (IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, p. 1624.) These interferometers overcome the complexity of design and fabrication problems of the Mach-Zehnder arrangement, but none of these references disclose using the already-available structure of the interferometer in a feedback system to operate the interferometer at its maximum sensitivity.